1. Field of the Invention
The present invention generally relates to piston rings for internal combustion engines. In particular, the present invention relates to a top compression rings for such engines.
2. Description of Related Art
Internal combustion engines operate on alternating compression and expansion cycles, which cycles reflect a state of operation within a combustion chamber. During the compression cycle, the compression of an air and fuel mixture typically precedes an ignition of the air and fuel mixture. The ignition of the air and fuel mixture results in combustion of the air and fuel mixture and an accompanying expansion within the combustion chamber. Such ignition and combustion generally increases the temperature of the component surrounding the chamber in which the combustion occurs. The expansion is followed, or accompanied, by an exhaust cycle.
The compression and expansion is generally enabled by a piston that reciprocates within a cylinder bore. Because the diameters of the piston and the receiving cylinder bore differ, a sealing arrangement is necessitated. Accordingly, one or more circumferential grooves are provided within an upper end of the piston. To provide a seal, resilient rings are installed in these grooves, which rings have a slightly larger outside diameter than the piston. The rings generally bear directly against the cylinder wall and create a seal between the sides and surfaces of the piston ring groove and the cylinder wall.
Recognition of the Problem
With reference to FIGS. 1a and 1b, an earlier embodiment of a top compression ring and ring groove are illustrated in a partially cross-sectioned view. As was known, a piston 10 was arranged to reciprocate within a cylinder bore or sleeve 12 of a cylinder block 14. To maintain a substantially sealed combustion chamber, the piston carried a compression ring 16 within an upper ring groove 18. A second ring was also carried within a second ring groove to scrap lubricant from the cylinder bore 12 while the piston 10 reciprocated within the bore 12.
As illustrated in FIG. 1(a), a bottom face 20 of the ring groove 18 was machined to be substantially horizontal such that an angle .beta. was approximately zero (i.e., the surface was normal to a reciprocating axis A of the piston 10). A bottom face 22 of the ring 16 was processed to have a slight taper when the bottom face 22 of the ring 18 was not in contact with the bottom face 20 of the ring groove 18. The angle relative to horizontal of the bottom face is identified in FIG. 1(a) as .alpha..
A problem was identified in that the ring 16 and the groove 18 might properly seat under cool operating conditions or when the engine has not achieved an operating temperature. As the engine was operating and the components began absorbing heat, the components would expand relative to one another and deform slightly due to complex geometries and stresses induced by thermal expansion of the components. Thus, due to such thermal expansion, the configuration of the ring groove and the ring would change as the temperature changed and improper seating between the members would result. The improper seating would create an increase in friction and a corresponding increase in ring or groove wear. Because the ring or groove wear may diminish the useful life of either member and may have a deleterious effect on engine performance over time, a solution was needed.
With reference now to FIG. 1(b), the problem discussed directly above is illustrated therein in the context of a thermally expanded piston and ring. As depicted in exaggerated form, the thermal expansion of the ring groove 18 and ring 16 resulted in an improper seating between the two elements. Specifically, an angle .theta. would be created between the bottom faces 20, 22.
A potential solution to this thermal-expansion problem is depicted in FIGS. 1(c) and 1(d). Due to the similarity of elements, like elements will be given like reference numerals with suffixes of "a" for purposes of this discussion of the problem solved by the present invention. As illustrated therein, the bottom face 20a of the groove 18a has been machined to have a downwardly inclined face from outside to inside. Specifically, while the engine and its components are cold, the bottom face 20a of the groove slopes downward and inward at the angle of .theta., the angle of deflection from above, while the bottom face 22a of the piston ring is designed to be substantially horizontal (again, normal to a reciprocating axis A of the piston 10a). Upon operating and heating to operation temperatures, the bottom face 20a of the groove 18a is thermally expanded and deformed.
Thus, under full-load, the piston groove and the piston ring become properly seated together as a result of thermal expansion. Accordingly, better combustion and gas sealing properties result. This solution, however, suffers from several drawbacks. For instance, due to the complicated geometry of the ring grooves that must be machined in every piston for every engine, manufacturing costs are high. Moreover, the particular ring groove deflection to be anticipated will vary with materials from which the pistons are manufactured due to differing coefficients of thermal expansion between materials and as a result of differing operating temperatures. To a lesser extent, one problem with any piston ring and piston groove fitting is simply the difficulty associated with verifying that the combination of angles and materials has been properly determined (i.e., it is difficult to visually inspect).